1. Field
One embodiment of the invention relates to a perpendicular magnetic recording medium and a magnetic recording apparatus equipped with the perpendicular magnetic recording medium.
2. Description of the Related Art
For increasing recording density of a hard disk drive (HDD), the size of one bit on a magnetic recording medium needs to be reduced. In order to permit signals to be correctly read even when the bit size is reduced, the size of magnetic grains constituting a magnetic recording layer need to be reduced. However, when the grain size is reduced, energy necessary for holding the recorded magnetization direction is reduced in proportion to the volume of the grain. When the energy is close to the thermal energy at room temperature, magnetization may be reversed without recording with a magnetic head (thermal fluctuation phenomenon).
For preventing this phenomenon, it may be considered to increase energy necessary for magnetization reversal per unit volume (uniaxial magnetic anisotropy energy; Ku). In this case, since the head magnetic field necessary for recording is basically proportional to Ku, sufficient recording becomes impossible unless the head magnetic field is increased in response to the increase of Ku. However, since saturation magnetic flux density of the magnetic head is currently almost close to a physical limit as well as the pole tip size of the magnetic head is also reduced as the bit size is reduced, it has been substantially difficult to enhance recording ability of the head.
Thus, in the present situation, an attempt to reduce the grain size for enhancing the recording density brings about difficulty in recording. Therefore, such an approach has been required to reduce the magnetic field necessary for recording while thermal fluctuation resistance is enhanced.
Recently, a solution referred to as a composite media has been proposed which has a columnar grain comprising a hard magnetic (high Ku) region and a soft magnetic (low Ku) region in which the two regions are coupled through appropriate intensity of interaction (see R. H. Victora et al., IEEE Transactions of Magnetics, Vol. 41, p. 537). Upon applying an external magnetic field, two regions are not simultaneously reversed in this configuration because two regions are not directly exchange-coupled. That is, the magnetization of the soft magnetic region starts to be rotated, and after it is rotated by a certain degree, it exerts a force on the hard magnetic region through an appropriate interaction so that the magnetization the hard magnetic region is rotated, by which the two regions are reversed. When two regions are directly exchange-coupled, two regions are simultaneously reversed at a magnetic field (coercivity) corresponding to an averaged Ku upon application of an external magnetic field. It has been reported that, however, if the exchange coupling is appropriately weakened, the two regions are reversed at a smaller magnetic field (coercivity) than that mentioned above. There are many reports on the actually fabricated composite media and evaluation of characteristics thereof (see, for example, J. P. Wang et al., IEEE Transactions on Magnetics, Vol. 41. P. 3181). All the reports are based on the original proposal, and thus a soft magnetic region is used as one of the two regions.
The coercivity may be certainly reduced in the case where the coupling between two regions is appropriately weakened as compared with the case where two regions are directly coupled, but the degree of reduction in coercivity depends on Ku and saturation magnetization Ms of respective regions rather than the strength of the coupling. In this case, roughly speaking, the coercivity may be determined mainly by the average value of Ku weighted with the layer thicknesses.
When the coercivity (reversal magnetic field) is set to a maximum in the recordable range under the assumption that the head magnetic field has already been at an upper limit, the average value of Ku in the composite media is almost automatically determined, and therefore, it becomes important how the each Ku value is assigned to two regions.
From the concept of the so-called composite media, if Ku of the soft magnetic region is approximately zero, then Ku of the hard magnetic region can be twice as much the value where there is no soft magnetic region. In other words, the composite media enables to use a hard magnetic layer having higher Ku. However, the average Ku of the composite media is made considerably lower than Ku of the hard magnetic layer.
On the other hand, in a CoCrPt-oxide granular hard magnetic layer which is a main stream of the currently available perpendicular HDD media, Ku can be increased by reducing Cr, but SNRm (signal-to-noise ratio of the media) can be improved as well as Ms can be reduced by increasing Cr. Thus, the Cr composition is made rather higher and Ku is controlled in the order of 4×106 erg/cc. Though use of such a material as FePt alloy providing quite high Ku have been considered, this alloy is not expected to provide as high SNRm as a CoCrPt recording layer. Under the circumstances, it is substantially difficult to increase Ku of the hard magnetic layer.
Besides the above perpendicular magnetic recording media, those using two magnetic layers in the magnetic recording layer have been proposed (see Jpn. Pat. Appln. KOKAI Publication Nos. 2003-168207 and 2006-48900). However, these media have not reached to have high thermal stability as well as reduced recording magnetic field and improved signal-to-noise ratio.